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The physical search for missing yachtie Alan Langdon and 6-year-old daughter Que has been called off by police who say they can't discount the possibility he has fled to Australia.

Experienced seaman Langdon, 49, and his daughter left Waikato's Kawhia Harbour bound for the Bay of Islands on December 17 and have not been seen or heard from since.

Border alerts are in place and Interpol has been working with New Zealand police, while last week the Herald reported a top child recovery specialist enlisted to find them said he thought they'd set sail for Australia.

Today, Sergeant Vincent Ranger of Waikato Police told the Herald that the search - which has scoped the entire west coast from Wellington to Cape Reinga, and down the east coast to the Bay of Islands - has been called off.

"We have searched all the areas we can physically search with the resources available," he said.

"We're still appealing to members of the public for any information or sightings.

"That they may be headed overseas is just one of many possible scenarios, and we're keeping an open mind.

"Our job is made more difficult by the fact that Mr Langdon didn't lodge an official trip report. As such, at this stage our focus has been on searching New Zealand waters."

Ranger said police "can't discount anything at this point".

It's possible that Langdon is headed for Australia, but to do so would seem "foolhardy", Ranger said.

General view of the township and habour at Kawhia on the Waikato coastline, where local man Alan Langdon and his daughter, Que Langdon, departed. Photo / Alan Gibson

"Our interest and focus is on locating Mr Langdon and his daughter to ensure their safety."

The massive search operation has included an RNZAF P3 Orion, Philips Search and Rescue Trust's fixed wing aircraft and helicopter, Northland Coastguard air patrol, Coastguard boats from Houhora and Hokianga, and commercial aircraft which have searched the southern coast of the North Island.

Ranger said police on the ground are involved in following up various lines of inquiry. Langdon's bank accounts and phone records have also been checked.

"Police continue to call for any sightings of the boat or Mr Langdon and his daughter," he said.

"We have been searching and making inquiries regarding the most obvious possible scenarios, but without having more information as to where they may have been headed, we're calling on the public to help us."

Que's mother, Ariane Wyler, told the Herald that she separated from Langdon last year.

Wyler - who lives at Golden Bay in the South Island but is currently in her native Switzerland - has now hired Australian child recovery specialist Col Chapman to help find her daughter who she has not seen for 20 months.

"I am deeply distressed about this current situation and miss my daughter Que greatly," she said.

"I am strongly convinced that they both are alive, well and safe. I believe in Alan Langdon and his capabilities as a seagoing person. My gut feeling tells me that they are further offshore to the west."

Chapman arrives in New Zealand later today to begin his inquiries on the ground.

However, Chapman thinks Langdon has already made a dash for Australia, and latest calculations suggest he could already be in Australian waters.

Langdon's 6m white wooden Tiki catamaran, which has blue anti-fouling paint under the waterline, is modified with a Bermuda mast, and sail ID number of T878.

Chapman today said that the latest information garnered from experienced professional sailors lead to conclusions he could already be across the Tasman Sea.

"This catamaran has turned out to be quite a remarkable boat," Chapman said.

"It's extremely reliable, it handles brilliantly apparently, better than any other 6m catamaran. It depends on the weather every day and whether he is sailing hands on, or by auto-pilot... but it all contributes to the fact he could be here now."

Langdon and his daughter are travelling by catamaran. Photo / Supplied

Chapman says he has been receiving clear, concise, and frequent" updates from New Zealand authorities.

"It's a big trip but once he crosses a line a couple of hundred kilometres offshore [of Australia], he looks like a coastal cruiser. He doesn't look like he's come from New Zealand in that little thing."

The pair could then easily hide in Australia, Chapman said.

Chapman says it's not the first time that Langdon has gone on the run.

In March last year the Langdon family of three - Langdon, Que and Wyler - was in Port Vila, Vanuatu, when Cyclone Pam struck.

The category 5 severe tropical storm battered the island, claiming up to 16 lives and sinking at least 20 boats.

It sank their 46-foot catamaran Sanyasin and the Langdons were lucky to escape with their lives.

After the cyclone, when the Australian and New Zealand air forces were repatriating people, Langdon took off for Australia with Que, Chapman says.

Chapman was enlisted by Wyler to find Langdon who kept moving about.

He says he eventually tracked him to the New South Wales town of Nimbin where Langdon and Que were living in a campervan on a large farm.

It's understood that Langdon and Que moved to Kawhia in August where they lived with local woman, Mary Smith.

She said the Langdon family had used her home as a base for years in between travels.

Langdon, a stay-at-home dad, built the catamaran the pair left Kawhia in on Smith's front lawn.

Smith raised the alarm on Boxing Day after Langdon and Que had been at sea without word for nine days.

She described Langdon as a competent boatsman who doted on his daughter.